1. Filed of the Invention
The present invention relates to a Doppler radar speed detecting method, i.e. a method for detecting a speed based on the Doppler effect, and apparatus for implementing such a method, which are for use with an anti-skid controller mounted on an automobile, for example.
2. Prior Art
Apparatus implementing such methods, mounted on an automobile, for example, will direct an electric wave such as a microwave or a millimeter wave, or an acoustic wave from an antenna mounted on the automobile toward the surface of a road, and detect the Doppler frequency (fd) corresponding to the speed of the automobile from the reflected waves from the road surface. In fact, however, the beam from the antenna will diverge with an angle .theta., as shown in FIG. 2a. As shown in FIG. 2b, the Doppler signal consequently includes various frequency components whose amplitude and phase will vary depending on the state of the road surface.
FIG. 2a illustrates the known principle of the Doppler radar. In FIG. 2a: .phi. represents the angle between the center axis of the radiated beam and a direction perpendicular to a road surface;
V represents the speed, expressed in meters per second, of the automobile; and
.lambda. represents the wave length, expressed in meters, of the radiated microwave The theoretical Doppler frequency fd (also referred to as "theoretical frequency") is expressed as follows: EQU fd=2V/.lambda. sin.phi.(Hz),
and therefore, fd is proportional to "V sin.phi.".
FIG. 2b shows the distribution of electric power for different frequency components of the Doppler signal obtained from the microwave reflected by the road surface. Typically, the distribution may be a Bell-shaped distribution centered at the theoretical frequency fd with the divergence caused by the angle .theta..
In the actual apparatus, however, the speed of an automobile is calculated by an electronic circuit at regular intervals of 0.1 sec. while the automobile is running. This limits the sampling time before the calculation and thus limits the amount of data concerning the reflected wave which can be taken by the apparatus. FIG. 3 and 4 show histograms which schematically represent the number of occurrences of data for Doppler frequency regions, wherein any given value of the Doppler frequency on the X-axis o abscissa is represented as the distance from the ordinate.
The number of occurrences Doppler frequencies as plotted in FIG. 3 exhibits a distribution which has a maximum at the theoretical frequency fd and is symmetrical with respect to the maximum. However, such a distribution occurs when the sampling period is sufficiently long and therefore the number of sampled data is sufficiently large. When the sampling period is short and therefore the number of sampled data is small, however, the maximum is not necessarily equal to the theoretical frequency fd and the distribution may be distorted, as shown in FIG. 4. Speed detectors used for controlling various operations associated with the running of an automobile such as, for example, anti-skid controllers, need to have a quick response Therefore, because the number of sampled data is thus limited, prior art methods of calculating the Doppler frequency would produce an output including an error. These prior art methods have operated by counting the number of pulses during the sampling period, which pulses are obtained by converting the Doppler signal into the pulses by means of comparators, or by obtaining the mean value of data about pulse periods.
Another problem with these prior art methods have occurred when the reflecting surface is smooth, for example when the road surface is covered with a film of water due to rain, for example. This causes additional low frequency reflections. Therefore, the distribution of occurrence of Doppler frequencies extends into low frequency regions (to the left), as shown in FIG. 5. The above-mentioned prior art methods then produce an output including an increased error.
Japanese published unexamined patent application No. 57-194371 corresponding to U.S. Pat. No. 4,527,160 discloses a method of reducing the error by detecting the maximum value of the spectrum, on the assumption that the number of occurrence of Doppler frequencies should have their maximum at the theoretical frequency fd. When the amount of period data about the Doppler signal is not sufficient, the maximum value of the frequencies of occurrence is not necessarily equal to the theoretical frequency fd, and therefore such a method cannot serve as effective means where a quick response is required.
In Japanese published unexamined patent application No. 58-39971, a predetermined number of time periods, each corresponding to N times the wavelength of the reflected wave which is frequency-shifted under the Doppler effect, are collected as data and those ones of the data which are clearly out of the speed range of the moving object are discarded. Where errors due to variations of the period data within the speed range of the moving object become a problem, as previously described in conjunction with FIG. 4, this method cannot reduce the errors. In accordance with this method, effective ones of said time period data are further collected a predetermined number of times, and the mean value and standard deviation of these values are calculated. Those data which are out of the range between the mean value minus the standard deviation and the mean value plus the standard deviation are excluded, and the mean value of only the remaining data is obtained. Thus if the number of data is in sufficient, a statistically significant sample cannot be obtained and the signal processing cannot substantially reduce the errors. In particular, for data like that shown in FIG. 5, this method can reduce the errors only mininally, if at all.